Smooth Pigweed (Amaranthus hybridus (syn: quitensis)) is a dicot weed in the Amaranthaceae family. In Argentina this weed first evolved multiple resistance (to 2 herbicide sites of action) in 2016 and infests Soybean. Multiple resistance has evolved to herbicides in the Groups G/9, and O/4. These particular biotypes are known to have resistance to 2,4-D, dicamba, and glyphosate and they may be cross-resistant to other herbicides in the Groups G/9, and O/4.

The 'Group' letters/numbers that you see throughout this web site refer to the classification of herbicides by their site of action. To see a full list of herbicides and HRAC herbicide classifications click here.

Greenhouse trials comparing a known susceptible Smooth Pigweed biotype with this Smooth Pigweed biotype have been used to confirm resistance. For further information on the tests conducted please contact the local weed scientists that provided this information.

Genetics

Genetic studies on Group G, O/9, 4 resistant Smooth Pigweed have not been reported to the site. There may be a note below or an article discussing the genetics of this biotype in the Fact Sheets and Other Literature

Mechanism of Resistance

The mechanism of resistance for this biotype is either unknown or has not been entered in the database. If you know anything about the mechanism of resistance for this biotype then please update the database.

Relative Fitness

There is no record of differences in fitness or competitiveness of these resistant biotypes when compared to that of normal susceptible biotypes. If you have any information pertaining to the fitness of multiple resistant Smooth Pigweed from Argentina please update the database.

The Herbicide Resistance Action Committee, The Weed Science Society of America, and weed scientists in Argentina have been instrumental in providing you this information. Particular thanks is given to Pedro Christoffoleti, Eduardo Cortés, Rafael De Prado, and Ignacio Dellaferrera for providing detailed information.

The evolution of herbicide‐resistant weeds is one of the most important concerns of global agriculture. Amaranthus hybridus L. is a competitive weed for summer crops in South America. In this article, we intend to unravel the molecular mechanisms by which an A. hybridus population from Argentina has become resistant to extraordinarily high levels of glyphosate.

Results

The glyphosate‐resistant population (A) exhibited particularly high parameters of resistance (GR50 = 20 900 g ai ha−1, Rf = 314), with all plants completing a normal life cycle even after 32X dose application. No shikimic acid accumulation was detected in the resistant plants at any of the glyphosate concentrations tested. Molecular and genetic analyses revealed a novel triple substitution (TAP‐IVS: T102I, A103V, and P106S) in the 5‐enol‐pyruvylshikimate‐3‐phosphate synthase (EPSPS) enzyme of population A and an incipient increase on the epsps relative copy number but without effects on the epspstranscription levels. The novel mechanism was prevalent, with 48% and 52% of the individuals being homozygous and heterozygous for the triple substitution, respectively. In silico conformational studies revealed that TAP‐IVS triple substitution would generate an EPSPS with a functional active site but with an increased restriction to glyphosate binding.

Conclusion

The prevalence of the TAP‐IVS triple substitution as the sole mechanism detected in the highly glyphosate resistant population suggests the evolution of a new glyphosate resistance mechanism arising in A. hybridus. This is the first report of a naturally occurring EPSPS triple substitution and the first glyphosate target‐site resistance mechanism described in A. hybridus.

The introduction of glyphosate-resistant (GR) crops revolutionized weed management; however, the improper use of this technology has selected for a wide range of weeds resistant to glyphosate, referred to as superweeds. We characterized the high glyphosate resistance level of an Amaranthus hybridus population (GRH)—a superweed collected in a GR-soybean field from Cordoba, Argentina—as well as the resistance mechanisms that govern it in comparison to a susceptible population (GSH). The GRH population was 100.6 times more resistant than the GSH population. Reduced absorption and metabolism of glyphosate, as well as gene duplication of 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) or its overexpression did not contribute to this resistance. However, GSH plants translocated at least 10% more 14C-glyphosate to the rest of the plant and roots than GRH plants at 9 h after treatment. In addition, a novel triple amino acid substitution from TAP (wild type, GSH) to IVS (triple mutant, GRH) was identified in the EPSPS gene of the GRH. The nucleotide substitutions consisted of ATA102, GTC103 and TCA106 instead of ACA102, GCG103, and CCA106, respectively. The hydrogen bond distances between Gly-101 and Arg-105 positions increased from 2.89 Å (wild type) to 2.93 Å (triple-mutant) according to the EPSPS structural modeling. These results support that the high level of glyphosate resistance of the GRH A. hybridus population was mainly governed by the triple mutation TAP-IVS found of the EPSPS target site, but the impaired translocation of herbicide also contributed in this resistance..

ABSTRACT - The recent introduction of Palmer amaranth (Amaranthus palmeri) in Brazilian
agricultural areas may promote several changes on weed management, especially in no-till
systems and in glyphosate-resistant crops, since glyphosate-resistant biotypes of A. palmeri
have been frequently selected in other countries. Therefore, this research was developed in
order to evaluate the glyphosate susceptibility of a Palmer amaranth biotype recently identified
in the State of Mato Grosso, Brazil. For this purpose, glyphosate susceptibility of three
Amaranthus biotypes was compared: A. hybridus var. patulus, collected in the State of Rio
Grande do Sul - Brazil; A. hybridus var. patulus, collected in the State of São Paulo - Brazil;
and A. palmeri, collected in the State of Mato Grosso - Brazil. Dose-response curves were
generated for all biotypes, considering eight rates of glyphosate and six replicates. All the
experiments were repeated twice. Both A. hybridus biotypes were satisfactorily controlled by
glyphosate, demanding rates equal to or lower than 541.15 g a.e. ha-1 for 80% control (LD80).
The A. palmeri biotype was not controlled by glyphosate in any of the assessments and required
rates greater than 4,500 g a.e. ha-1 to reach LD80, which are economically and environmentally
unacceptable. Comparison of the Brazilian A. palmeri biotype to the A. hybridus biotypes, as
well as, to the results available in scientific international literature, led to the conclusion
that the Brazilian Palmer amaranth biotype is resistant to glyphosate..

A previously unknown glyphosate resistance mechanism, amplification of the 5-enolpyruvyl shikimate-3-phosphate synthase gene, was recently reported in Amaranthus palmeri. This evolved mechanism could introgress to other weedy Amaranthus species through interspecific hybridization, representing an avenue for acquisition of a novel adaptive trait. The objective of this study was to evaluate the potential for this glyphosate resistance trait to transfer via pollen from A. palmeri to five other weedy Amaranthus species (Amaranthus hybridus, Amaranthus powellii, Amaranthus retroflexus, Amaranthus spinosus, and Amaranthus tuberculatus). Field and greenhouse crosses were conducted using glyphosate-resistant male A. palmeri as pollen donors and the other Amaranthus species as pollen recipients. Hybridization between A. palmeri and A. spinosus occurred with frequencies in the field studies ranging from <0.01% to 0.4%, and 1.4% in greenhouse crosses. A majority of the A. spinosus × A. palmeri hybrids grown to flowering were monoecious and produced viable seed. Hybridization occurred in the field study between A. palmeri and A. tuberculatus (<0.2%), and between A. palmeri and A. hybridus (<0.01%). This is the first documentation of hybridization between A. palmeri and both A. spinosus and A. hybridus..

The incidence and wide spread of herbicide resistant weeds is a global problem. Over the past 65 years, repeated use of herbicides has resulted in the evolution of resistant weed species. The first resistant species to triazine was discovered in 1970 in the United States. Since then, a large number of weed species has evolved resistance to several classes of herbicide. Currently, there are 334 resistant biotypes, including 190 weed species (113 dicots and 77 monocots) in over 310, 000 fields around the world Common resistant species are Chenopodium album and Amaranthus retroflexus resistant to triazine, Phalaris minor resistant to isoproturon, P. minor and P. paradoxa resistant to diclofop, Echinochloa colona resistant to propanil, Echinochloa crusgalli resistant to butachlor, Eleusine indica resistant to trifluralin, Lolium rigidum resistant to diclofop, Lactuca serriola resistant to metsuljuron, glyphosate resistance to Eleusine indica, Conyza canadensis, Lolium rigidum, and Lolium multiflorum. Multiple weed resistance to more than one class of herbicides with different modes of action has also been documented with many species. Currently there has been increased herbicide resistance to various weed species around the globe. Most common species are Lolium rigidum, Avena fatua, Amaranthus retroflexus, Chenopodium album, Setaria viridis, Echinochloa crusgalli, Eleusine indica, Kochia scoparia, Conyza canadensis, and Amaranthus hybridus..